Powder coating composition, process for the production and use thereof

ABSTRACT

Powder coating composition obtainable by homogeneous mixing of a separately produced particulate component 
     A prepared from one or more epoxy resins having an epoxy equivalent weight of 250 to 2500, one or more curing components together with conventional lacquer auxiliary substances and additives and optionally pigments and/or fillers and 
     a separately produced particulate component 
     prepared from one or more resins differing from A having an acid value of 20 to 70 and/or an OH value of 10 to 100, one or more curing components together with conventional lacquer auxiliary substances and additives and optionally pigments and/or fillers, 
     in a mixing ratio of component A to component B of 1:99 to 50:50, relative to weight.

This application is the national phase of international applicationPCT/EP98/06199 filed Sep. 30, 1998 which designated the U.S.

FIELD OF THE INVENTION

This invention relates to powder coating compositions produced using aparticular process for coating metal components. The metal componentsmay be not only individual. components and sheets, but also finishedmanufactured products.

BACKGROUND OF THE INVENTION

Metal surfaces, for example of aluminium and other non-ferrous metalsand of steel, are provided with coatings, increasingly with powdercoating compositions, for corrosion protection purposes. The metalsurface must be finished for coating in such a manner that it ensuresadequate adhesion of the powder coating. This is achieved, for example,by pretreating the metal surface with agents which cause the depositionof a microscopic layer of crystals, so passivating the surface withregard to corrosion and providing keying points for the powder coatingmaterial. Such pretreatment methods comprise, for example, chromatingand phosphating, in particular zinc or iron phosphating.

In addition to aluminium and the alloys thereof as well as bright steel,galvanised steel is increasingly being used with a coating of corrosion-and weathering-resistant powder coatings. The galvanised steel may beproduced by both the hot-dip and the electrogalvanisation processes. Theparticular advantage of using galvanised steel is that the zinc coatingalone continues to provide protection from corrosion if the powdercoating film is damaged. A higher level of corrosion protection may beachieved by such zinc coatings than is achieved by the above-statedpassivation pretreatment of ungalvanised metal surfaces.

However, powder coating films do not adhere adequately to galvanisedmetal surfaces. In order to provide adhesion, galvanised metal surfacesmust, for example, be pretreated with phosphating solutions, for exampleaccording to DE-A-21 53 809, FR-A-26 81 33, WO 92/15727, and are thencoatable using a powder coating, for example based on polyester, epoxy,acrylate and alkyd resins.

It is furthermore possible initially to coat the galvanised steelsurface with a primer, before subsequent coating operations areperformed, c.f. for example the use of a powder primer based on an epoxyresin and an aromatic diamine according to JP-A 61 250 067.

The above-stated pretreatment methods require the additional processingsteps of priming and phosphating and/or chromating and thus associatedadditional steps such as rinsing, drying, coating, stoving. Suchprocesses are accordingly time-consuming and cost-intensive and, in thecase of chromating, environmentally hazardous.

Steel or zinc surfaces may furthermore, for example, be coated with anepoxy powder coating substantially consisting of a bisphenol A epoxyresin and a phenolic crosslinking agent for subsequent coating with alacquer based on a fluorocarbon resin according to JP-A-01 040 329.Potential applications include, for example, coloured roofing.

Powder coating compositions are known, the use of which does not requirepretreatment, for example phosphating pretreatment. According to U.S.Pat. No. 3,882,064, rapid-curing powder coating compositions containingas binder an epoxy resin mixture together with polyethylene wax anddicyandiamide may be used on galvanised steel. Such coating compositionsare suitable as protective coatings for containers for foodstuffs andbeverages and are not intended for weathering-resistant coatings.

EP-A-0 057 334 describes coating galvanised metal with a powder coating,in which the metal is cleaned with acid and then rinsed before coating.During cleaning, for example using low concentration solutions ofphosphorous acid, under certain temperature conditions the zinc surfaceundergoes only very slight attack accompanied by the formation of verysmall quantities of zinc phosphate crystals. Powder coatings based onepoxy, polyester and acrylic resins and mixtures thereof may be used asthe coatings. Hybrids which are mentioned are, for example, mixtures ofpolyester and epoxy resins. The objective is to ensure the slightestpossible attack on the zinc coating which provides corrosion protectionwhile still ensuring good adhesion of the powder coating by rapidcoating after drying. However, when used in exterior applications, thesecompositions are susceptible to weathering, especially on long-termexposure to the action of the weather.

Known hybrid powders, for example mixtures of polyester and epoxy resinsin a 50:50 ratio, are produced using conventional powder coatingtechnology by mixing all the components of a powder formulation, such asbinder, curing agent, additives and pigments and fillers, in a dry mixerfor a mixing time optimised for the type of mixer. The resultant mixedmaterial (premix) is homogenised, for example extruded, by means of anextruder under temperature and rotational speed conditions optimised forthe particular formulation, cooled using appropriate cooling means andpre-comminuted using a crusher. This pre-comminuted intermediate productis then finely ground in appropriate classifying mills to the grain sizedistribution adapted to the intended application. If such hybrid powdersare applied onto metal substrates which have not been chemicallypretreated, for example galvanised steel, corrosion protection problemsmay arise due to detachment of the powder film. Known hybrid powders ofthe above-stated composition moreover cannot be used as weatherresistant coating systems.

SUMMARY OF THE INVENTION

The object of the present invention is accordingly to provide a specificcoating material for metals and a specific process for the production ofthis coating material, such that good corrosion protection combined withgood adhesion of the coating material on the metal surface may beachieved by application of a single layer and moreover such that,depending upon requirements, very good weather resistance is achievable,so combining several important lacquer properties in only one layer tobe applied.

It has been found that this object may be achieved by a powder coatingcomposition which is obtained by a selected processing method.

The present invention accordingly provides a powder coating compositionobtainable by homogeneous mixing of a separately produced particulatecomponent

A prepared from one or more epoxy resins having an epoxy equivalentweight of 250 to 2500, one or more curing components together withconventional lacquer auxiliary substances and additives and optionallypigments and/or fillers and

a separately produced particulate component

B prepared from one or more resins differing from A having an acid valueof 20 to 70 and/or an OH value of 10 to 100, one or more curingcomponents together with conventional lacquer auxiliary substances andadditives and optionally pigments and/or fillers,

in a mixing ratio of component A to component B of 1:99 to 50:50,relative to weight.

The present invention also provides a process, in which a powder coatingcomposition which contains

A one or more epoxy resins having an epoxy equivalent weight of 250 to2500, one or more curing components together with conventional lacquerauxiliary substances and additives and optionally pigments and/orfillers and

B one or more resins differing from A having an acid value of 20 to 70and/or an OH value of 10 to 100, one or more curing components togetherwith conventional lacquer auxiliary substances and additives andoptionally pigments and/or fillers

in a mixing ratio of component A to component B of 1:99 to 50:50, isproduced in such a manner that component A and component B are initiallyproduced separately using conventional powder coating productionprocesses and the two components A and B in the stated mixing ratio arethen subjected to a further operation, for example an extrusionoperation, to ensure homogeneous mixing of the two components.

The content of epoxy resin in component A may, for example, be in arange between 10 wt. % and 95 wt. %.

The curing component is present in conventional quantities, generally ina quantity suitable for reaction with functional groups of the resincomponent in order to achieve the desired degree of crosslinking.

Depending upon requirements, the epoxy resin may be replaced by otherresins, for example epoxy novolak, phenolic and/or melamine resin and byresins which contain epoxy groups.

With regard to component B. the resin may, for example, be presenttherein in a quantity of 20 wt. % to 95 wt. %.

The curing agent content corresponds to the conventional quantities, ingeneral a quantity suitable for reaction with functional groups of theresin component in order to achieve the desired degree of crosslinking.

Component B resins may, for example, be polyester resins, (meth)acrylateresins and modified copolymers. Polyester resins and/or (meth)acrylateresins are preferably used.

According to the invention, component A and component B are producedseparately using conventional powder coating production methods, forexample by mixing, extrusion, cooling, comminution optionally thenfollowed by grinding to conventional grain sizes.

According to the invention, components A and B are in particulate form.“Particulate” should be taken to mean that components A and B are usablein powder form or in the form of a cooled, comminuted extrudate whichhas not been ground to conventional grain sizes. The cooled andcomminuted extrudate may be coarsely and irregularly divided anddescribed as being in “chip” form.

The resultant components A and B are thereupon mixed to yield aso-called dryblend mixture, wherein the ratio of component A tocomponent B is 1:99 to 50:50. This mixture is subjected to a furtherhomogenisation operation. Grinding to conventional powder grain sizes isthen performed.

The further homogenisation operation for mixing the two components A andB may comprise any suitable process for homogeneously mixing the twocomponents.

The components are preferably homogenised as a melt.

This may, for example, be achieved by extruding the two components A andB. To this end, components A and B are, for example, intensivelypremixed in dry form and then melted in an extruder, for example attemperatures of 70 to 130° C. and intensively mixed. The resultantextrudate is rolled out into thin layers, cooled and crushed to formcoarse pellets, which may be ground in a mill to the desired grain sizeof the powder coating composition according to the invention, forexample in a grain size range of 1 to 100 μm.

Components A and B may furthermore be homogenised by intensive mixingand melting of the components and then spraying them as a melt.

Another option for performing homogenisation from a melt comprises, forexample, using a low molecular weight inert compound in the form ofcompressible fluids, for example carbon dioxide, as an auxiliarysubstance. In this case, the powder coatings A and B may be dissolved inor homogeneously mixed with the supercritical fluid and the resultantmixture depressurised by spraying. A and B are preferably homogeneouslymixed as a melt, for example by extrusion.

Depending upon the required range of properties, mixtures of component Aand component B in a ratio of 1:99 to 10:90 may be used for coatingsystems having very good weathering resistance and good adhesion orcorrosion protection, while mixtures in a ratio of 50:50 to 15:85 may beused for coating systems having very good adhesion or corrosionprotection and good weather resistance (in each case relative toweight).

The powder coatings used are based on known binder systems. Thesecomprise, for example, epoxy, polyester and/or (meth)acrylate basedresins optionally together with further modified copolymers. The termused here denotes “acrylic and/or methacrylic”.

The component A epoxy powder coatings contain epoxy resins as theprincipal binder component. Examples of epoxy resins are reactionproducts prepared from epichlorohydrin with bisphenol, for examplebisphenol A.

Conventional curing agents may be used for the epoxy resins. Epoxyresins often crosslink by means not only of those curing agentscontaining carboxyl groups, but also those containing amide or aminogroups. Curing agents which may, for example, be used are dicyandiamideand the derivatives thereof, carboxylic acids or the anhydrides thereof.

The epoxy resin may be entirely or partially replaced by further resinssuch as epoxy novolak, phenolic resin, melamine resin and by resinswhich contain epoxy groups. Phenolic resins based on phenol andformaldehydes, as are for example known as resols or novolaks, may, forexample, be used. Examples of melamine resins are butanol- and/ormethanol-etherified melamines.

Polyester powder coating compositions in which the binder constituentcomprises polyesters containing carboxyl and/or hydroxyl groups may beused as component B.

Polyesters containing hydroxyl groups are condensation products preparedfrom aliphatic, aromatic and/or cycloaliphatic polycarboxylic acids andpolyalcohols.

Polycarboxylic acids which may, for example, be used are phthalic acid,1,4-cyclohexanedicarboxylic acid, tetrahydrophthalic acid, adipic acid,sebacic acid, terephthalic acid, trimellitic anhydride and in generalthe anhydrides and esters thereof. Examples of polyalcohols are ethyleneglycol, diethylene glycol, propylene glycol, hexanediol, neopentylglycol, propanediol, trimethylolpropane, pentaerythritol, neopentylglycol ester of hydroxypivalic acid. Mixtures thereof may also be used.Carboxy-functionalised polyesters may likewise be produced by reactingpolycarboxylic acids and glycols, but with an excess of acids.

Mixtures of polyesters containing carboxyl and/or hydroxyl groups mayalso, in general, be used.

Curing agents which may be used are conventional curing agents, such asfor example cycloaliphatic, aliphatic or aromatic polyisocyanates,crosslinking agents containing epoxy groups, such as for exampletriglycidyl isocyanurate (TGIC), polyglycidyl ethers based on diethyleneglycol, glycidyl-functionalised (meth)acrylic copolymers as well ascrosslinking agents containing amino, amido or hydroxyl groups.

Component B may also contain (meth)acrylate resins and modifiedcopolymers as a binder constituent. Modified copolymers may beconventional copolymers with modifications, such as epoxy groups,carboxyl groups, hydroxyl groups, isocyanate groups and/or amide groups.They may, for example, each contain two of the stated functional groups.These resins may, for example, comprise modified (meth)acrylate resins,such as copolymers prepared from alkyl (meth)acrylates with glycidyl(meth)acrylates and olefinic monomers, such as styrene and/or styrenederivatives. They may also comprise modified vinyl copolymers, forexample based on monomers containing glycidyl groups and one or moreethylenically unsaturated monomers, for example, alkyl (meth)acrylate,styrene, styrene derivatives, (meth)acrylamide or also grafted vinylcopolymers which are, for example, grafted with ethylenicallyunsaturated acids, ethylenically unsaturated acid derivatives or theanhydrides thereof.

The component B resins may be present individually or as mixtures.

Conventional curing agents are usable, such as for example soliddicarboxylic acids, for example having 10 to 12 carbon atoms, as well ascarboxy-functional polymers.

Conventional quantities of inorganic and/or organic pigments, fillersand/or conventional lacquer auxiliary substances and additives may beadded in order to produce components A and B according to the invention.Conventional lacquer auxiliary substances and additives compriseconventional lacquer additives as are usual in the lacquer sector.Quantities are within the conventional range familiar to the personskilled in the art. Components A and B according to the invention may,for example, each contain 0 to 50 wt. % of one or more pigments and/orfillers. The quantity of additives may, for example, be from 0.01 to 10wt. %.

Examples of conventional auxiliary substances and additives forcomponents A and B are levelling agents, degassing agents, thixotropingagents and accelerators and others. Levelling agents which may be usedare, for example, (meth)acrylic polymers, (meth)acrylic copolymers,silicon copolymers, thixotroping agents which may be used are forexample silicon compounds and accelerators which may be used are, forexample, amino, amide and imidazole compounds, anhydrides and organicsalts. Further conventional lacquer additives may furthermore beincorporated into the compositions of components A and B. These areadditives which act, for example, to improve scratch resistance,adhesion, UV resistance, dispersibility, gloss and are familiar to theperson skilled in the art.

A component A according to the invention may, for example, contain 40 to70 wt. % of epoxy resin, 2 to 30 wt. % of curing agent, 0 to 50 wt. % offiller and 0 to 50 wt. % of inorganic and/or organic pigments,optionally together with further additional substances and additives.

Component B may, for example, contain 40 to 70 wt. % 6 of polyesterresin, 2 to 30 wt. % of curing agent, 0 to 50 wt. % of filler and 0 to50 wt. % of inorganic and/or organic pigments, optionally together withfurther additional substances and additives.

Production of components A and B and mixing of the resultant componentsA and B may proceed using the conventional powder production processesdescribed above, for example by extrusion processes.

Once ground using known grinding units, the powder coating compositionproduced according to the invention may have a conventional grain sizedistribution of 1 to 200 μm, preferably of up to 100 μm.

The powder coating composition according to the invention may be appliedonto metal surfaces and stoved using conventional powder coatingmethods. Usable metals are, for example, aluminium and the alloysthereof, further non-ferrous metals together with steel and galvanisedsteel. These powder coating systems are stoved in accordance with thetemperature and time conditions required for complete crosslinking.Stoving may, for example, be performed at object temperatures of 100° C.to 250° C. for a period of 40 to 1 minute.

Depending upon the requirements placed upon the coated object, chemicalpretreatment or priming of the metal surface may be required.

The powder coating systems according to the invention are suitable foruse in exterior applications on metal surfaces which are exposed tolong-term weathering. The powder coating compositions are, for example,suitable for metal surfaces such as cladding elements, fencing systems,steel structures, garden equipment, motor vehicle components.

The powder coating composition according to the invention provides goodadhesion and corrosion resistance combined with weathering resistance onmetal surfaces, it frequently being possible to omit conventionalpretreatment or priming. Depending upon requirements, powder coatingsystems according to the invention may purposefully be produced whichhave very good weather resistance and good corrosion protection, forexample in cladding elements, or with good weather resistance and verygood corrosion protection, for example in steel structures, especiallythose exposed to industrial atmospheres, by the combination according tothe invention of component A and component B. Two or more importantdesired lacquer properties may thus purposefully be combined in only onelayer to be applied, whereas multilayer coatings are conventionallyrequired to achieve the desired lacquer properties.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following Examples are intended to illustrate the invention:

EXAMPLE 1 Production of Component A

500 parts by weight of an epoxy resin having an epoxy equivalent weightof 1500, 20 parts by weight of dicyandiamide, 10 parts by weight ofsilicone-based levelling agent, 150 parts by weight of barium sulfateand 200 parts by weight of titanium dioxide are mixed and then extrudedin a conventional extruder at a temperature of 80 to 130° C. Thecomposition is then cooled and comminuted and then ground to an averagegrain size of 20 to 80 μm.

EXAMPLE 2 Production of Component B

500 parts by weight of a polyester resin having an OH value of 50 (COOHvalue of 40), 37 parts by weight of a conventional curing agent, 6 partsby weight of a conventional accelerator, 10 parts by weight of asilicone-based levelling agent, 150 parts by weight of barium sulfateand 200 parts by weight of titanium dioxide are mixed, extruded,comminuted and ground to an average grain size of 20 to 80 μm as inExample 1.

EXAMPLE 3 Testing of Weather Resistance and Corrosion Resistance

Components A and B produced by way of example according to Examples 1and 2 are mixed in the following ratios (relative to parts by weight):

1. A:B=5:95

2. A:B=10:90

3. A:B=30:70

4. A:B=50:50

These mixtures are homogenised in an extruder at a temperature of 80 to130° C. and then ground to a grain size range of 1 to 100 μm. Theresultant powder is applied onto a degreased steel sheet to a filmthickness of 60 to 100 μm and stoved for 10 minutes at an objecttemperature of 200° C.

The table shows the test results relating to the behaviour of the powdercoating layer on the metal surface (corrosion protection and weatherresistance). These results are compared with the results obtained onapplication of a conventional polyester powder coating (table, item 1),epoxy powder (table, item 2), hybrid powder coating (items 3 and 4according to the table) and on application of a two-layer powdercoating, in which the first layer consists of an epoxy powder coatingand the second layer of a polyester powder coating (c.f. table, item 9).

Test results from the salt spray test to DIN 50021 (1000 hours) and theQUVB rapid weathering apparatus (250 hours) revealed that thecompositions according to the invention according to items 5 and 6exhibit good weather resistance simultaneously combined with goodcorrosion resistance, in particular in the case of mixtures ofcomponents A and B in a ratio of 5:95 to 10:90. These test results werevirtually comparable with a two-layer structure according to item 9 inthe table. These results are surprising because, when used as the solecoating substance, the components A and B used in the formulation orhybrid powder coatings are not capable of combining the two essentialproperties, very good corrosion protection and very good weatherresistance, in a single material on metal substrates which have not beenspecially pretreated.

In contrast, coatings based on conventional hybrid powders according toitems 3 and 4 exhibit poorer gloss values and were notcorrosion-resistant.

If, after separate extrusion, components A and B according to Examples 1and 2 are dry mixed as a powder in the A:B ratio according to theinvention of 1:99 to 50:50, but are not homogenised again according tothe invention, and the resultant powder mixture is applied in theconventional manner under the above-stated conditions, adequate weatherresistance is indeed achieved, but adequate adhesion of the coating onexposure to mechanical stress is not, and thus only slight corrosionprotection or adequate corrosion protection combined with only slightweather resistance is achieved.

Corrosion test to DIN 50031 on degreased steel QUVB exposure (creepageto light; from score residual gloss after 1000 h, after 250 h, % mm) 1Polyester powder >50 completely coating (as B) detached after 240 h 2Epoxy powder coating 3 12 (as A) 3 Conventional hybrid 12 completelypowder B:A = 50:50 detached after 240 h 4 Conventional hybrid 20completely powder B:A = 70:30 detached after 240 h 5 Invention B:A =95:5 >50 16 6 Invention B:A = 90:10 50 14 7 Invention B:A = 70:30 25 128 Invention B:A = 50:50 15 12 9 2 layers: >50 12 1^(st) powder A, 2^(nd)powder B

What is claimed is:
 1. A powder coating composition comprising: aparticulate component (A) prepared from an epoxy resin prepared fromepichlorohydrin and bisphenol having an epoxy equivalent weight of 250to 2500 and at least one resin from the group consisting of epoxynovolak resins, phenolic resins and melamine resins, or prepared from atleast one epoxy resin having an epoxy equivalent weight of 250 to 2500and at least one resin selected from group consisting of epoxy novolakresins, phenolic resins, and melamine resins, at least one curingcomponent, conventional lacquer auxiliary substances and additives, andoptionally, one or more ingredients selected from the group consistingof a pigment and a filler; a particulate component (B) prepared from atleast one resin selected from the group consisting of polyester resins,(meth)acrylate resins and modified copolymers, the at least one resin ofparticulate component (B) differing from the at lest one epoxy resin ofparticulate component (A), the at least one resin of component (B)having an acid value of 20 to 70, an OH value of 10 to 100, or acombination thereof, at least one curing component, conventional lacquerauxiliary substances and additives, and optionally, one or moreingredients selected from the group consisting of a pigment and afiller, wherein particulate component (A) and particulate component (B)are prepared separately by mixing, extruding, cooling and grinding; andwherein particulate component (A) and particulate component (B) aremixed in a ratio of 1:99 to 50:50, relative to weight, and homogenizedby melting in an extruder.
 2. A method for using the powder coatingcomposition according to claim 1, comprising: coating metal surfaceswith said powder coating composition.
 3. A powder coating compositionaccording to claim 1, wherein particulate component (A) and particulatecomponent (B) are mixed in a ratio of particulate component (A) toparticulate component (B) of 1:99 to 10:90.
 4. A powder coatingcomposition according to claim 1, wherein particulate component (A) andparticulate component (B) are mixed in a ratio of particulate component(A) to particulate component (B) of 50:50 to 15:85.
 5. A process forproducing a powder coating composition, comprising: producing aparticulate component (A) prepared from an epoxy resin prepared fromepichlorohydrin and bisphenol having an epoxy equivalent weight of 250to 2500 and at learn one resin selected from the group consisting ofepoxy novolak resins, phenolic resins and melamine resins, or preparedfrom at least one epoxy resin having an epoxy equivalent weight of 250to 2500 and at least one resin selected from the group consisting epoxynovolak resins, phenolic resins, and melamine resins, at least onecuring component, conventional lacquer auxiliary substances andadditives, and optionally one or more ingredients selected from thegroup consisting of a pigment and a filler by mixing, extruding, coolingand grinding; producing particulate component (B) prepared from at leastone resin selected from the group consisting of polyester resins,(meth)acrylate resins and modified copolymers, the at least one resin ofparticulate component (B) differing from the at least one epoxy resin ofparticulate component (A), the at least one epoxy resin of component (B)having an acid value of 20 to 70, an OH value of 10 to 100, or acombination thereof, at least one curing component conventional lacquerauxiliary substances and additives, and optionally, one or moreingredients selected from the group consisting of a pigment and afiller, by mixing, extruding, cooling, and grinding; mixing particulatecomponent (A) an particulate component (B) in a ratio of 1:99 to 50;50,relative to weight, and homogenizing particulate component (A) anparticulate component (B) together by melting in an extruder.
 6. Amethod for using a powder coating composition produced according to theprocess of claim 5, comprising: coating metal surfaces with said powdercoating composition.